Superfluids and friction

No longer a mere liquid, the helium has become a superfluid—a liquid that flows without friction. "If you set [down] a cup with a liquid circulating around and you come back 10 minutes later, of course it's stopped moving," says John Beamish, an experimental physicist at the University of Alberta in Edmonton. Atoms in the liquid will collide with one another and slow down. "But if you did that with helium at low temperature and came back a million years later," he says, "it would still be moving."

Why wouldn't it? For a large bulk at low temperature, the probability of vortices tunneling from the core to the edge is easily low enough that it won't happen on the lifetime of the universe. In fact, I would be surprised if it wasn't some absurdly large number times the lifetime of the universe.

No longer a mere liquid, the helium has become a superfluid—a liquid that flows without friction. "If you set [down] a cup with a liquid circulating around and you come back 10 minutes later, of course it's stopped moving," says John Beamish, an experimental physicist at the University of Alberta in Edmonton. Atoms in the liquid will collide with one another and slow down. "But if you did that with helium at low temperature and came back a million years later," he says, "it would still be moving."

One of the most common questions regarding superconducting magnets, especially those used at the LHC, is why not cool the liquid helium to an even lower temperatures to increase the rate of heat absorption/transport, and to increase the current. People forget that if you lower the temperature even more and LHe becomes a superfluid, you cannot pump on it to cause it to flow from one location to another! If it stays in one location, it will stay in one location! The lack of viscosity simply will not allow the bulk fluid to "move with the masses" the way normal fluid does.

So what they end up doing (and I believe this is what is done at the LHC based on my conversation with people there), is to have a 2-phase mixture of LHe. The ordinary LHe is pumped on and provides the heat flow to various parts of the superconducting magnet, whereas the superfluid He resides in various locations where it is "trapped" and provide the added, more efficient heat transfer. So it is almost the scenario where the ordinary LHe exchanges heat with the superfluid LHe.

A more fundamental property than the disappearance of viscosity becomes visible if superfluid is placed in a rotating container. Instead of rotating uniformly with the container, the rotating state consists of quantized vortices. That is, when the container is rotated at speed below the first critical velocity (related to the quantum numbers for the element in question) the liquid remains perfectly stationary. Once the first critical velocity is reached, the superfluid will very quickly begin spinning at the critical speed. The speed is quantized, i.e. it can only spin at certain speeds. In basic terms, if the container is rotated to a certain speed, the superfluid will rotate very quickly along with the container, otherwise, if the speed is too slow, then the superfluid will not move at all, unlike how a normal fluid like water will rotate along with its container from the start.